1. Field of the Invention
The present invention relates generally to semiconductor memory devices, and more particularly, a method of and an apparatus for reducing current flow from a power supply into a memory device immediately after the power supply is turned on.
2. Description of the Prior Art
FIG. 1 is a block diagram showing a memory board generally employed in apparatuses utilizing computers. Referring to FIG. 1, the memory board 40 comprises a number of memory chips 1 for storing data signals and a control circuit 41 for controlling the memory chip 1. The memory chip 1 is connected to receive voltage from an external power supply Vcc.sub.1 through a terminal 42 and the control circuit 41 is connected to receive a voltage from another external power supply Vcc.sub.2 through a terminal 43. The control circuit 41 generates a RAS (Row Address Strobe) signal, a CAS (Column Address Strobe) signal and address signals, and controls reading/writing of the memory chip 1 based on instructions from a CPU (Central Processing Unit).
In order to supply power to the memory chip 1 and the control circuit 41, different power supplies Vcc.sub.1 and Vcc.sub.2 such as shown in FIG. 1 are utilized, or a common power supply is utilized. For example, if a backup power supply for a memory chip 1 is used, two different supply voltages are applied. In either case, the level (high or low) of the RAS signal applied to the memory chip 1 depend on the system when the power is turned on.
FIG. 2 is a block diagram showing a conventional 1 M-bit dynamic RAM (Random Access Memory Device). The dynamic RAM such as shown in FIG. 1 is disclosed in "A reliable 1-M bit DRAM with a multi-bit-test mode" by M. Kumanoya et al., 1985 (IEEE Journal Solid-State Circuits, vol. SC-20, pp. 909-913) and also in "A Fast 256K.times.4 CMOS DRAM with a Distributed Sense and Unique Restore Circuits" by H. Miyamoto et al., 1987 (IEEE Journal Solid-State Circuits, vol. SC-22, pp. 861-867).
Referring to FIG. 2, the dynamic RAM comprises a clock generator circuit 10 for outputting clock signals .phi..sub.1 and .phi..sub.2 which control this dynamic RAM in response to a CAS signal and a RAS signal. The CAS signal and the RAS signal are externally applied through a CAS terminal 8 and a RAS terminal 4, respectively. A power supply Vcc (5 V) and the ground Vss (0 V) are externally applied through a power supply terminal 2 and a ground terminal 3, respectively.
FIG. 3 is a timing chart showing the change of the current to be consumed in the dynamic RAM. Referring to FIG. 3, the dynamic RAM has two states of operation, that is, standby state and active state. The dynamic RAM is brought to the standby state when a high level RAS signal is applied, while it is brought to the active state when a low level RAS signal is applied. As is apparent from the figure, the current Icc to be consumed flowing from the power supply (Vcc shown in FIG. 2) changes dependent on the state of operation of the dynamic RAM.
In the standby state, an approximately constant current I.sub.2 of about 1.about.3 mA flows from the power supply Vcc to the dynamic RAM. (The reason for this will be described later.)
Immediately after the change of the RAS signal from high level to low level, the dynamic RAM is brought to the active state and a transient current I.sub.a flows. The current I.sub.a mainly comprises a charging current for activating the clock generator circuit 10 and an operating current for operating the row address buffer 21 and the row decoder 22 in FIG. 2. After 30.about.50 n sec from the change of the RAS signal to the low level, a transient current I.sub.b flows. The current I.sub.b is consumed by the sense amplifier 24 to charge bit lines in the memory array 25. The bit line charging operation by the sense amplifier 24 in the active state will be described in detail later.
When the current I.sub.b is decreased, a constant current I.sub.4 flows to activate the data output buffer 27. The current I.sub.4 is less than 10 mA in a normal state.
Thereafter, immediately after the change of the RAS signal from the low level to the high level, the dynamic RAM returns to the standby state and a transient current I.sub.c flows. The current I.sub.c mainly comprises a current for bringing the clock generator circuit 10 to the standby state and a current for bringing the row address buffer 21 and the row decoder 22 to the standby state.
FIG. 4 is a schematic diagram showing the clock generator circuit of the dynamic RAM in FIG. 2. Referring to FIG. 4, the clock generator circuit 10 comprises a buffer circuit connected to a RAS terminal 4 and a inner circuit 11 connected between a power supply Vcc and the ground Vss for outputting clock signals .phi..sub.1 and .phi..sub.2 in response to a signal from the buffer circuit. The buffer circuit comprises two inverters 5a and 5b connected in series. A reference character Icc represents consumed current flowing from the power supply Vcc to a dynamic RAM 1.
In general, for a circuit receiving an input signal from the outside, a buffer circuit connected to an input terminal comprises inverters. For example, a buffer circuit employing inverters is described by Neil H. E. Weste et al. in "PRINCIPLES OF CMOS VLSI DESIGN", pp. 227-229, published by ADDISON-WESLEY PUBLISHING COMPANY in 1985.
A description is made of operation which occurs when the supply voltage Vcc is externally applied to the dynamic RAM in FIG. 4.
FIGS. 5 and 6 are timing charts showing a change of signals for explaining the operation of the dynamic RAM in FIG. 4. Referring to FIGS. 5 and 6, the supply voltage Vcc starts to be applied to the terminal 2 from a time t.sub.1 and the applied voltage rises up to a predetermined voltage level. When the applied voltage reaches the predetermined voltage level, it will not change thereafter.
FIG. 5 shows the case in which a high-level RAS signal is applied to the RAS terminal 4 before the time t.sub.1. The dynamic RAM is in the standby state when the RAS signal is at high level and it is in the active state when the RAS signal is at low level. Power consumption is small when the dynamic RAM is in the standby state and it is large when the dynamic RAM is in the active state. Therefore, the supply voltage Vcc (for example 5 V) is applied to the dynamic RAM while the dynamic RAM is in the standby state in FIG. 5. As a result, after the current Icc flowing into the dynamic RAM reaches its small peak value of I.sub.1 (several mA) at a time t.sub.2, the value is reduced to I.sub.2 which is smaller than I.sub.1, and then stabilized. The value of I.sub.2 is a current value necessary for operation in the standby state. The reason why these different values flow will be described in the following.
FIG. 7 is a schematic diagram showing a buffer circuit in the clock generator circuit 10 shown in FIG. 4. Referring to FIG. 7, the buffer circuit comprises two inverters 5a and 6a. The inverter 5a comprises a series connection of a P channel MOS transistor Q1 and an N channel MOS transistor Q2 connected between the power supply Vcc and the ground Vss. The gates of the transistors Q1 and Q2 are connected together and the RAS signal is applied thereto. The inverter 5b also comprises a P channel MOS transistor Q3 and an N channel MOS transistor Q4 connected in a similar manner as the inverter 5a. The gates of the transistors Q3 and Q4 are connected together to the output of the inverter 5a. A stray capacitance C10 exists between the output node N10 of the inverter 5a and the ground Vss, and a stray capacitance C11 exists between the output node N11 of the inverter 5b and the ground Vss.
FIG. 8 is a timing chart showing the change of the voltage at output nodes of two inverters shown in FIG. 7 when the power supply Vcc rises. Referring to FIGS. 7 and 8, the nodes N10 and N11 are at 0 V before the voltage of the power supply Vcc rises. When the power supply Vcc rises after a high level RAS signal is applied, the output node N10 of the inverter 5a remains at 0 V. Meanwhile, the output node N11 of the inverter 5b is brought to a high level voltage, so that the stray capacitance C11 existing between the node N11 and the ground Vss is charged. Therefore, a charging current flows from the power supply Vcc.
Various peripheral circuits are provided in the dynamic RAM as shown in FIG. 2, each of which comprising, in most cases, circuits such as shown in FIG. 7. As described above, immediately after the power supply Vcc is turned on, charging currents for charging stray capacitances in these circuits flow in, causing a peak current I.sub.1 at the time t.sub.2 shown in FIG. 5.
Referring again to FIG. 7, the current I.sub.2 of a constant value consumed after the time t.sub.2 will be described. The current I.sub.2 corresponds to the current I.sub.2 from the power supply Vcc which is shown in FIG. 3.
Generally, the RAS signal has a voltage level called TTL (Transistor Transistor Logic) level. More specifically, the high level of the RAS signal is about 2.4 V when the power supply Vcc is 5 V. The transistor Q2 turns on in response to a high level RAS signal applied between the gate and the source thereof. Meanwhile, the transistor Q1 receives approximately -2.6[=-(Vcc-2.4)]V between the gate and the source thereof, and turns on. Therefore, both transistors Q1 and Q2 turn on and a current flows from the power supply Vcc to the ground Vss. This current is included in the current I.sub.2 shown in FIG. 5, which flows constantly. In addition, a current which will be described in the following is also included in the current I.sub.2.
FIG. 9 is a schematic diagram showing a ring oscillator provided for generating negative voltage in the dynamic RAM. Referring to FIG. 9, the ring oscillator comprises an odd-number of inverters 29 which is connected in series to form a ring. A pulsating current which fluctuates in several mega-hertz frequency flows into the ring oscillator from the power supply Vcc. Since this current is of high frequency, it seems as a direct current and is included in the current I.sub.2 shown in FIG. 5.
On the other hand, the timing chart of the FIG. 6 shows the case in which the supply voltage Vcc starts to be applied to the dynamic RAM from the time t.sub.1, while the RAS signal is low level. Since the supply voltage Vcc is applied to the RAM chip while the RAM chip is in the active state, the current Icc after the time t.sub.1 is increased. At this time, since each node of circuits in the dynamic RAM has not been necessarily brought to a predetermined high or low level, excessive current Icc flows therein. As a result, after the current Icc reaches its big peak value of I.sub.3 (several tens of mA) which is bigger than the value of I.sub.1 at the time t.sub.3, it is reduced to the value of I.sub.4 (below 10 mA) which is considerably smaller than the value of I.sub.3, and then stabilized. The value of I.sub.4 is a current value necessary for operation in the active state, which is the same as that shown in FIG. 3.
A description is made of the reason for the inflow of the excessive current hereinafter.
FIG. 10 is a schematic diagram showing an example of portions of the sense amplifier 24 and the memory array 25 of the dynamic RAM shown in FIG. 2. Referring to FIG. 10, the sense amplifier 24 comprises two latch circuits connected between a bit line 241 and a bit line 242. One latch circuit is constituted by N channel MOS transistors Q10 and Q11 and is connected to the ground Vss through an N channel MOS transistor Q12. The other circuit is constituted by P channel MOS transistors Q13 and Q14 and is connected to the power supply Vcc through a P channel MOS transistor Q15. The gates of the transistors Q12 and Q15 are connected such that they receive sense signals .phi..sub.s and .phi..sub.s respectively, which signals are inverted from each other.
The memory array 25 is connected to the sense amplifier 24 through the bit lines 241 and 242. Memory cells MC each consisted of one N channel MOS transistor and a capacitor are connected between the bit line 241 or 242 and the word line 243. There are stray capacitances C.sub.B1 and C.sub.B2 between respective bit lines 241 and 242 and the ground Vss.
FIG. 11 is a timing chart showing the operation of a circuit shown in FIG. 10 when the power supply Vcc rises after a high level RAS signal is applied (in this case, it corresponds to the case shown in FIG. 5). Referring to FIGS. 10 and 11, the bit lines 241 and 242 are at 0 V before the power supply Vcc rises. When a high level RAS signal is applied and the power supply Vcc rises, a sense signal .phi..sub.s of 0 V is applied to the gate of the transistor Q12. Therefore, the transistor Q12 remains off. Meanwhile, a sense signal .phi..sub.s which goes to a high level from 0 V simultaneously with the rise of the power supply Vcc is applied to the gate of the transistor Q15. Therefore, the transistor Q15 also remains off. Since both transistors Q12 and Q15 are off, the stray capacitances C.sub.B1 and C.sub.B2 are not charged. That is, the bit lines 241 and 242 are not charged by the power supply Vcc, and no current flows in from the power supply Vcc.
FIG. 12 shows a timing chart in which the power supply Vcc rises while the RAS signal remains at low level (corresponding to the case shown in FIG. 6). Referring to FIGS. 10 and 12, the bit lines 241 and 242 are at 0 V before the rise of the power supply Vcc. A sense signal .phi..sub.s which has risen to a high level from 0 V simultaneously with the rise of the power supply Vcc is applied to the gate of the transistor Q12. Therefore, the transistor Q12 turns on. Meanwhile, a sense signal .phi..sub.s of 0 V is applied to the transistor Q15, and the transistor Q15 also turns on. Since both transistors Q12 and Q15 are turned on, current flows into the bit lines 241 and 242 from the power supply Vcc through the transistor Q15 and to the ground Vss through the transistor Q12. The voltages at the bit lines 241 and 242 are slightly increased from 0 V due to this current. On this occasion, a through current flows from the power supply Vcc to the ground Vss through the transistor Q15, Q13 or Q14, Q10 or Q11, and Q12.
Thereafter, since the sense amplifier 24 comprises two latch circuits as described above, the bit line 241, for example, is brought to a high level and the bit line 242 is brought to a low level. Which of the two bit lines 241 and 242 is brought to the high level is determined by a slight imbalance between the stray capacitances C.sub.B1 and C.sub.B2 having approximately the same capacitance value. Since one of the two bit lines 241 and 242 is charged by the power supply Vcc, a charging current flows into the dynamic RAM from the power supply Vcc. Generally, one stray capacitance C.sub.B1 or C.sub.B2 has a value less than 0.4 pF. Therefore, in a case of 1 mega-bit dynamic RAM, for example, 2048 stray capacitances are charged, with the total capacitance value being 819 pF (=0.4 pF.times.2048). A current for charging the total capacitance is included in the current I.sub.3 shown in FIG. 6.
The current I.sub.3 shown in FIG. 6 comprises the following current besides the above described through current and the charging current from the power supply Vcc. Referring again to FIG. 7, when the power supply Vcc rises with the RAS signal being low level, the output node N10 of the inverter 5a is brought to a high level voltage from 0 V. Therefore, the stray capacitance C10 existing between the node N10 and the ground Vss is charged by the power supply Vcc and a charging current flows in from the power supply Vcc. As described above, the dynamic RAM comprises a number of circuits such as shown in FIG. 7 and such charging currents are included in the current I.sub.3 shown in FIG. 6.
Meanwhile, the constant current I.sub.4 which flows after the time t.sub.3 corresponds to the current I.sub.4 of the timing chart shown in FIG. 3.
As described above, in the conventional dynamic RAM, the excessive current I.sub.3 (for example 50 mA) from the power supply Vcc flows in when the power supply Vcc is turned on. Because of this excessive current I.sub.3, power supply capacity could be insufficient, so that other circuits could not operate correctly or the line fuse could be blown. In addition, the heat produced by this excessive current could cause a malfunction of the dynamic RAM.